(a) Field of the Invention
The present invention relates to a display device.
(b) Description of Related Art
Recent trends of light-weight and thin personal computers, televisions sets that require light-weight and thin display devices, and flat panel displays satisfying such a requirement are being substituted for conventional cathode ray tubes (CRT).
The flat panel displays include a liquid crystal display (LCD), field emission display (FED), organic light emitting diode (OLED) display, plasma display panel (PDP), and the like.
Generally, an active matrix flat panel display includes a plurality of pixels arranged in a matrix and displays images by controlling the luminance of the pixels based on given luminance information. An OLED display is a self-emissive display device that displays images by electrically exciting light emitting organic material, and has low power consumption, wide viewing angle, and fast response time, thereby being advantageous for displaying motion images.
A pixel of an OLED display includes an OLED and a driving thin film transistor (TFT). The TFT includes polysilicon or amorphous silicon. A polysilicon TFT has several advantages, but it also has disadvantages such as the complexity of manufacturing polysilicon, thereby increasing the manufacturing cost. In addition, it is hard to make an OLED display employing polysilicon TFTs be large.
On the contrary, an amorphous silicon TFT is easily applicable to a large OLED display and manufactured by less number of process steps than the polysilicon TFT. However, the threshold voltage of the amorphous silicon TFT shifts over time due to a long-time application of a unidirectional voltage to a gate of the TFT such that the current flowing in the OLED is non-uniform, the image quality is degraded and the lifetime of the OLED is shortened.
An OLED display, in particular, a bottom emission OLED display, includes a transparent anode made of indium tin oxide (ITO), etc., a light emitting layer, and an opaque cathode, which are sequentially formed on a glass substrate. In this structure, the anode, boundary surfaces of the substrate, and the cathode form a light guide that prevents about 50% of the emitted light from escaping. About 30% of the emitted light is destructed due to the total reflection at the substrate surface between the substrate and the air. Therefore, the theoretical efficiency of the outward light emission is equal to about 20%.
In addition, an anti-reflection film used for improving contrast ratio may drop the actual light emission efficiency below about 10%.